Question: A box contains $6$ red marbles, $3$ green marbles, and $9$ blue marbles. If a marble is randomly chosen, what is the probability that it is blue?
Solution: There are $6 + 3 + 9 = 18$ marbles in the box. There are $9$ blue marbles. The probability is $ \frac{9}{18} = \dfrac{1}{2}$.